There are a number of diseases and disorders which only manifest their presence in sleep or at least markedly worsen during sleep. One sleep linked disorder is obstructive sleep apnea. A patient suffering this disorder undergoes while asleep repeated episodes of obstruction of their upper airway. During these episodes of obstruction there is a period, which can typically range between 10-40 seconds, when there is no airflow through the airway into the lungs of a patient. This state usually terminates due to arousal of the patient.
In mild cases of sleep apnea, a sufferer may only undergo a few episodes of obstruction over a night of 6-8 hours of sleep. In more severe cases, the episodes can occur repeatedly with tests showing that some sufferers undergo 400-500 episodes of obstruction in a single night's sleep. Such high levels of obstruction and the resulting repeated falls in oxygen level to the brain can lead to major health problems, and sufferers are known to have increased mortality, probably because of increased morbidity due to cardiovascular diseases and stroke. Sufferers also exhibit high levels of daytime sleepiness which increases the risk of a sufferer being involved in a traffic or industrial accident. It is now recognised that at any one time about 10% of males and about 5% of females suffer from some form of sleep apnea.
Obstructive sleep apnea is now widely recognized by its array of daytime symptoms including daytime sleepiness and also by witnessed episodes of "stop breathing" by a bed partner. Heavy snoring is, however, the most common and characteristic sign of sleep apnea.
Snoring is almost universally known and is characterised by noise generated in the throat of a sleeping individual. Snoring is in fact so tightly coupled with sleep that its presence is a positive and diagnostic indication that an individual is asleep.
Snoring is a noise which is typically generated by vibration of the air conduction tube at the level of the throat. It usually occurs during inspiration (breathing in) but sometimes occurs during expiration (breathing out) and sometimes during both phases of the sleeping cycle. Snoring occurs when, with sleep, there is a loss of muscle tone in the muscles of the throat which are needed to keep the throat open. This loss in muscle tone narrows the air passages leading to inadequate levels of airflow into the lungs. This drop in airflow is detected and the body's reflexes automatically produce increased efforts to breathe. These increased efforts produce greater degrees of suction pressure in the air passages which then in turn cause the walls of the throat to vibrate and flutter. Snoring commonly originates in the oropharyngeal area of the throat. This is the region where the cavity of the mouth and a cavity from the nasal airway join to form one tube at the oral region of the pharynx. In this region, the soft palate which is a mobile structure which hangs down from the roof of the mouth in the back of the throat, acts to determine which of the two pathways are connected to the breathing tube. Most commonly, snoring is generated when the soft palate flutters. This fluttering leads to pressure oscillations within the airway and sounds are then generated when other structures in and around the airway are vibrated producing harmonics which can be heard.
The underlying physical properties of snoring start, therefore, when the airway flutters at a critical location resulting in the suction pressure in the airway also oscillating. This pressure oscillation radiates in all directions, including both outwards through the nose and mouth, and inwards back into the lungs and chest wall. The pressure oscillation causes other tissues to oscillate and vibrate and harmonics are formed causing audible sound.
The audible sound which is widely known a snoring is the result of vibration of a range of tissues and the harmonics which are then formed. The sound is the result of complex movements end various filtering by nasal structures. In contrast, the underlying pressure oscillations of the airway which leads to these sounds are inaudible. The easiest way to demonstrate the existence of the underlying pressure oscillation is for an individual to voluntarily snore. This can be done with a little training; the throat is relaxed, the mouth opened, and then strong inspiratory efforts are made. Doing this, most people can make their soft palate flutter and thus generate snoring sounds. When the nose is observed during this action, the walls of the nasal entry can be seen to oscillate as a result of the low frequency inaudible pressure wave being transmitted along the airway.
While snoring is the most common and characteristic symptom of sleep apnea it does not necessarily follow that if a person is a heavy snorer they will also suffer sleep apnea. A report an heavy snoring can, therefore, alone not provide enough information to positively diagnose or exclude the existence of sleep apnea. When a subject develops sleep apnea, however, the pattern of snoring typically changes. Instead of the sound of regular rhythmical snoring occurring at a time sequence identical to the breathing cycle there are typically bursts of a few loud snoring breaths, separated by a longer period of silence of 15-50 seconds indicative of obstruction. This sequence usually occurs in a regularly occurring pattern over many minutes or even hours.
The recognition of this characteristic pattern of snoring has been used to diagnose the presence of adult sleep apnea. There are many devices which are based on the recording of sound and many of these are used as diagnostic devices to identify snoring. These devices range from microphones being placed in a room of a sufferer or on the throat or even in a hearing aid placed in the ear of a sufferer. Some devices use sound analysis systems to characterise the sound signature and then to count the number of snores or the number of apneas in a night of recording. Many doctors also ask their patients to record their own snoring and then return the tapes so that they may listen for the characteristic pattern of sounds. This method is very good as it utilizes the physician's car as the analysis system which can readily detect the typical sound of snoring and then the typical pattern of broken snoring indicative of sleep apnea. It is, however, an impractical method as the audio recordings have to be replayed and to date there has been no efficient way of providing quantitative analysis.
A particular problem with recording sound alone is that it is also difficult to separate other sounds such as someone talking, a door slamming, the radio or television in the background, a car or truck passing outside, or even someone else's snoring in the room. To counter this problem, an independent measure of respiration is typically used to timelock the sound of snoring to the breathing cycle. Typical devices for measuring respiration are thermistors and pressure transducers that are attached to the patient and measure airflow at the mouth or nose or other devices also attached to the patient which monitor chest movement.